The present invention relates to a brake force control apparatus and, more particularly, to a brake force control apparatus which executes a brake assist control that generates a brake force greater than that generated at an ordinary time, when an emergency braking is performed by a driver of a vehicle.
Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus is known which generates, when an emergency braking is required, a brake force greater than that generated in a normal time. The above-mentioned conventional apparatus is equipped with a brake booster, which generates a pushing force having a given power ratio with respect to a brake pressing force Fp. The pushing force generated by the brake booster is transferred to a master cylinder. The master cylinder generates a master cylinder pressure PM/C based on the pushing force of the brake booster, that is, the brake pressing force Fp.
The above-conventional apparatus is equipped with a fluid pressure generating mechanism, which generates an assist hydraulic pressure in which a pump is used as a fluid pressure source. The fluid pressure generating mechanism generates the assist hydraulic pressure based on a driving signal supplied from a control circuit. When the brake pedal is operated at a speed higher than a predetermined speed, the control circuit determines that an emergency braking is carried out by the driver, and requests a fluid pressure generating mechanism to the maximum assist hydraulic pressure. The maximum assist hydraulic pressure generated by the fluid pressure generating mechanism is supplied to a change valve together with the master cylinder pressure PM/C. The change valve supplies the higher one of the assist hydraulic pressure generated by the fluid pressure generating mechanism and the master cylinder pressure PM/C toward wheel cylinders.
According to the conventional apparatus, if the brake pedal is operated at a speed equal to or less than the given operating speed, the master cylinder pressure PM/C adjusted to a level depending on the brake pressing force Fp is supplied to the wheel cylinders. Hereinafter, the control of realizing the above-mentioned state will be referred to as a normal control. If the brake pedal is operated at a speed higher than the given operating speed, a high assist hydraulic pressure is supplied to the wheel cylinders in which the pump serves as a fluid pressure source. Hereinafter, the control of realizing the above-mentioned state will be referred to as a brake assist control Hence, according to the conventional apparatus, the brake force is controlled to a level based on the brake pressing force Fp at ordinary time, and to rapidly increase the brake force after emergency braking is executed.
The above-mentioned conventional apparatus is equipped with a change valve. The change valve is a mechanism, which selects a state in which the master cylinder serves as a fluid pressure source or a state in which the fluid pressure generating mechanism serves as a fluid pressure source. More particularly, the change valve selectively realizes a state in which the wheel cylinders are coupled to the master cylinder (hereinafter referred to as a first state) and a state in which the wheel cylinders are coupled to the fluid pressure generating mechanism (hereinafter referred to as second state).
The first and second states can also be realized by a two-position switch valve. The change valve has a complex structure and is expensive than the two-position switch valve. Hence, when the two-position switch valve is used instead of the change valve, the conventional apparatus can be produced at a reduced cost.
Anyway, the above-mentioned conventional apparatus starts to increase the wheel cylinder pressure at the same time as emergency braking is performed. However, if the switching between the fluid liquid sources is implemented by the two-position switch valve, the switching time is required to be determined taking into consideration the output characteristic of the fluid pressure generating mechanism and the motion performance of the vehicle.
More particularly. at the time when emergency braking is recognized, the brake pedal is being operated at a high speed, and thus a rapid increase in the master cylinder pressure PM/C takes place. The speed at which the assist hydraulic pressure generated by the fluid pressure source is limited to an appropriate value due to he capability of the pump or the like.
Hence, if the fluid pressure source is switched to the fluid pressure generating mechanism from the master cylinder immediately after the emergency braking is recognized. a decrease in the increasing speed of the wheel cylinder pressure, as compared to a case where the master cylinder is maintained as the fluid pressure source. In order to prevent occurrence such a problem at the time of emergency braking, it is desired that the fluid pressure source be switched to the fluid pressure generating mechanism from the master cylinder when an appropriate delay time elapses after the emergency braking is recognized.
In the above-mentioned apparatus, when the brake assist control is started, the ground contact ability of tires is partially consumed in order to produce brake force. Hence, if the brake assist control is executed while the vehicle is turned, the maximum value of cornering forces which can be produced by the tires becomes less than that a value obtained when the brake assist control is not executed.
The maximum value of the cornering forces generated in the tires increases as the load exerted on the tires increases. Hence, when the brake assist control is active in which the weight of the vehicle is shifted toward the front wheels, the maximum value of the cornering forces which can be produced by the rear tires is greatly decreased.
The above-mentioned conventional apparatus executes the brake assist control without any consideration of the turning state of the vehicle. Hence, in the vehicle equipped with the above-mentioned conventional apparatus, the maximum value of the cornering forces generated by the rear wheels may greatly be less than the maximum value of the cornering forces generated by the front wheels.
In order to suppress the decrease in the cornering forces of the rear wheels, it is desirable that the brake assist control to the rear wheels be started with an appropriate delay time after the brake assist control to the front wheels is started.
The conventional apparatus in which the brake assist control to the front wheels and that to the rear wheels are simultaneously started is not an ideal one capable of maintaining the stable behavior of the vehicle which is turning.
The present invention is made in view of the above-mentioned point, and it is the first object of the present invention to provide a brake force control apparatus in which the brake assist control is started at a timing different from the timing at which an emergency braking is recognized and preferable braking performance can thus be provided.
The brake force control apparatus directed to achieving the above object includes an operation fluid pressure generating mechanism that generates an operation fluid pressure depending on a degree of operation of a brake pedal by a driver, a high-pressure source generating a control fluid pressure higher than that of the fluid pressure generated by said operation fluid pressure generating mechanism, a switch mechanism for selectively connecting one of the operation fluid pressure generating mechanism to the high-pressure source and a wheel cylinder, and emergency braking detection means for detecting execution of an emergency braking. When an emergency braking is performed by the driver, a brake assist control for boosting a wheel cylinder pressure in such a way that the high-pressure source serves as a fluid pressure source
The brake force control apparatus which achieves the above object is configured so that, as a start timing, a time is detected at which a controlled pressure increasing slope obtained by boosting the wheel cylinder pressure with the high-pressure source used as the fluid pressure source exceeds a normal pressure increasing slope obtained by boosting the wheel cylinder pressure with the master cylinder used as the fluid pressure source. Also, the brake assist control is started after the emergency braking is detected and the start timing is then detected.
In the brake force control apparatus according to the present intention, a fluid pressure based on a brake operation force is supplied, at the normal time, to the wheel cylinders from the operation fluid pressure generating mechanism. After the brake assist control is started, the high-pressure source is used as the fluid source, and a fluid pressure higher than that at the normal time is supplied to the wheel cylinders in the present invention, the brake assist control employs, as the fluid pressure source, the operation fluid pressure generating mechanism, and starts after a situation is created in which the wheel cylinder pressure can rather be boosted rapidly with the high-pressure source used as the fluid pressure source Hence, it is possible to prevent an event in which an increase in the wheel cylinder pressure is prevented due to execution of the brake assist control.
A second object of the present invention is to provide a brake force control apparatus in which the brake assist control of the front wheels and the brake assist control of the rear wheels are started at respective, different times and preferable braking performance can be realized.
The brake force control apparatus which achieves the above object is equipped with a high-pressure source which generates a predetermined control fluid pressure, a front wheel fluid pressure control mechanism that controls a wheel cylinder pressure of front wheels with the high-pressure source used as a fluid pressure source, a rear wheel fluid pressure control mechanism that controls a rear wheel cylinder pressure of rear wheels with the high-pressure source used as the fluid pressure source, and emergency braking detection means for detecting execution of an emergency braking. When an emergency braking is performed by the driver, a brake assist control for generating a wheel cylinder pressure higher than that generated at a normal time is generated.
The brake assist control of the front wheels is started after the emergency braking is detected. Then the brake assist control of the rear wheels is started when a predetermined rear wheel delay time elapses after the brake assist control of the front wheel is started. The brake assist control of the rear wheels in the present invention is started when the predetermined rear wheel delay time elapses after the brake assist control of the front wheels is started. When the brake assist control of the front wheels is started, the vertical load of the rear wheels is decreased due to load shifting, and thus the cornering forces that can be generated by the rear wheels is reduced. Hence, if the brake assist controls of the front and rear wheels are simultaneously started, the cornering forces generated by the rear wheels may abruptly be decreased According to the present invention, it is possible to suppress decrease in the cornering forces of the rear wheels resulting from the start of the brake assist control. Hence, even if an emergency braking is performed when the vehicle is turning, a large braking force can be ensured without change of the turning behavior.